Evaluation of New Applications of Oil Shale Ashes in Building Materials

Article Evaluation of New Applications of Oil Shale Ashes in Building Materials

Mustafa Cem Usta 1,*, Can Rüstü Yörük 1, Tiina Hain 2, Peeter Paaver 3, Ruben Snellings 4, Eduard Rozov 5, Andre Gregor 1, Rein Kuusik 1, Andres Trikkel 1 and Mai Uibu 1 1 Department of Materials and Environmental Technology, Tallinn University of Technology, 19086 Tallinn, Estonia; [email protected] (C.R.Y.); [email protected] (A.G.); [email protected] (R.K.); [email protected] (A.T.); [email protected] (M.U.) 2 Department of Civil Engineering and Architecture, Tallinn University of Technology, 19086 Tallinn, Estonia; [email protected] 3 Department of Geology, University of Tartu, 50411 Tartu, Estonia; [email protected] 4 Sustainable Materials, VITO, 2400 Mol, Belgium; [email protected] 5 Wienerberger, 43401 Aseri, Lääne-Virumaa, Estonia; [email protected] * Correspondence: [email protected]

Received: 3 July 2020; Accepted: 27 August 2020; Published: 29 August 2020

Abstract: Achieving sustainable zero-waste and carbon neutral solutions that contribute to a circular economy is critically important for the long-term prosperity and continuity of traditional carbon-based energy industries. The Estonian oil shale (OS) sector is an example where such solutions are more than welcome. The combustion of OS generates a continuous flow of ashes destined to landfills. In this study, the technical feasibility of producing monolith building materials incorporating different OS ashes from Estonia was evaluated. Three binder systems were studied: self-cementation of the ashes, ceramic sintering in clay brick production and accelerated carbonation of OS ash (OSA) compacts. Results showed that most of the OSAs studied have low self-cementitious properties and these properties were affected by ash fineness and mineralogical composition. In case of clay bricks, OSA addition resulted in a higher porosity and improved insulation properties. The carbonated OSA compacts showed promising compressive strength. Accelerated carbonation of compacted samples was found to be the most promising way for the future utilization of OSAs as sustainable zero-waste and carbon neutral solution.

Keywords: oil shale ash; waste utilization; concrete; bricks; carbonation curing

1. Introduction New initiatives by the European Commission not only aim at reducing air and water emissions but also extensively encourage innovation in waste or residue recovery using “Best-Available Technologies” (BAT) that promote transitions towards green energy production under the principles of circular economy [1]. In this context, the Estonian oil shale (OS) sector is a good example where the management and utilization of waste is vital to ensure long term sustainability. Estonia is still mostly utilizing low calorific fuel—OS as a primary source of energy including electricity, heat and oil production across the country. This heavy fossil fuel reliance produces abundant amounts of uncommon calcareous ash which has been deposited in landfills and waste piles over the years, since the ash lacks industrial applications [2,3]. Historically landfilled ashes as well as the currently generated ashes carry important risks to the biosphere such as emissions of hazardous trace elements (Sr, Zr, As, Cd, Cu, Cr, Zn, Pb) as well as alkalinity to groundwater and air [4–6]. In recent decades the implementation of circulating fluidized bed (CFB) boilers for combustion and advanced retorting technologies for oil production have greatly increased extraction efficiencies

Minerals 2020, 10, 765; doi:10.3390/min10090765 www.mdpi.com/journal/minerals Minerals 2020, 10, 765 2 of 19 and reduced GHG emissions. However, these changes in the process are not without pitfalls as the physical and chemical characteristics of the OSA were negatively affected. For instance, lowering the temperatures from 1300–1400 ◦C (previously used in pulverized firing (PF) boilers) to 700–800 ◦C (used in current CFB boilers) changed the phase composition of the ashes, altered the content of unburnt materials and increased calcium sulfate contents. The phase composition shifted away from high-temperature Ca-silicates towards free lime and quartz. This complicates the utilization of OSA by inducing volumetric expansion and poses environmental issues by increasing the pH of OSAs [7]. The utilization of OSA in the production of new valuable products could be a partial solution for the Estonian OS sector by integrating core concepts from circular economy. In this respect, OSA as any other industrial alkaline solid waste (such as lignite, coal, wood bottom and fly ashes (FAs), steel slags, cement production wastes and waste concrete), can be considered as a valuable raw material in the conventional production processes of cement, concrete and ceramics [8–10]. Evidence of this utilization has existed previously in the Estonian context, where OSA, collected from electrostatic precipitators (EP) of PF units, was used as a raw material for the production of Portland clinker. Additionally, coarse fractions were used as aggregates in the production of cellular concrete blocks and in the applications of road-base stabilization [2,7]. However, due to the above-mentioned changes in the OS incineration process such applications have been phased out. Therefore, there is an urgent need to investigate alternative application routes for OSAs. The current study includes three sub-studies of oil shale ash utilization in building materials; first sub-study is testing of self-cementing properties of all the currently generated OSAs, second sub-study is the clay brick production with oil shale ash to test its performance as opening agent and the last sub-study is on the properties of OSA monoliths obtained by accelerated carbonation, which draws on the recent developments in research of carbonate bonded construction materials [11–13].

2. Materials and Methods

2.1. Self-Cementing Performance

2.1.1. Materials A range of OSAs were included in the present study. The selected ashes were mainly FAs regularly collected in the period of 2018 and 2019 including electrostatic precipitator ash (EPA), cyclone ash (CA), total ash (Mixture of all flow of ashes from PP, except EPAs) (TA) and mixtures of different FAs from the Auvere (A) and Eesti (E) power plants (PP). Additionally, CA and TA from the Enefit 280 oil production plant were included. In total, 6 different ash streams (EPP-EPA, EPP-TA, APP-EPA, APP-TA, EN280-CA, EN280-TA) were considered.

2.1.2. Material Characterization The physical, chemical and mineralogical characterization of the selected waste streams included determination of total carbon (TC) and total inorganic carbon (TIC) with an Eltra CS 580 (Haan, North Rhine-Westphalia, Germany) Carbon Sulphur Determinator and free CaO content (based on ethylene glycol method), X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses with Bruker S4 Pioneer (Karlsruhe, Baden-Württemberg, Germany) and Bruker D8 (Karlsruhe, Baden-Württemberg, Germany) diffractometers, respectively. For XRD analysis, randomly oriented preparations were made and scanned on a Bruker D8 Advance diffractometer using Cu Kα radiation with a Göbel mirror monochromator and LynxEye positive sensitive detector over a 2◦–70◦ 2Θ range. The quantitative phase composition was analysed and modelled using the Rietveld algorithm-based program Topas (Karlsruhe, Baden-Württemberg, Germany). The relative error of quantification is better than 10% for major phases (>5 wt. %) and better than 20% for minor phases (<5 wt. %). The BET-N2 sorption method was used to measure the specific surface area (SSA) with Kelvin 1042 sorptiometer. Minerals 2020, 10, 765 3 of 19

The particle size distribution (PSD) was measured by laser diﬀraction using a Horiba (Kyoto, Japan) Laser Scattering instrument, LA-950 (with ethanol suspension).

2.1.3. Sample Preparation In order to test the self-cementing properties of the OSAs, ash and sand pastes were prepared for all waste streams. 0.7 was found to be optimum ratio of water to ash [14] and ash to sand (cf. EN 196-1:2016) ratio was 0.33. Pastes were cast in 40 40 160 mm3 prisms and compacted using × × a vibration table. The compacted pastes were first kept 48 h in molds, then five days at 60% RH and 20 2 C, and further curing continued for 28 days at >95% RH and 20 2 C. After 28 days, ± ◦ ± ◦ the failure strength (flexural and compressive strength) was tested following EN 196-1:2016 [15]. The aim of the work regarding self-cementing properties of oil shale ashes was to find possibilities for utilization of oil shale ashes. These types of studies are based on our own experience with oil shale throughout the years. The methods described therein might vary from those utilized in the applications of Portland cements due to the distinct influence of OSA types and properties of pastes and mortars.

2.1.4. Tests and Measurements Flexural strength test determined the maximum bending stress of prisms before failure. This test was conducted on a Toni TechnikD-13355 (Berlin, Brandenburg, Germany) which works in accordance with EN ISO 7500-1 (2018) [16]. The compressive strength test was also performed using the same apparatus which applies a progressing load rate of 2400 N/S in accordance with EN 196-1:2016 and ISO 679 [17]. The split mortar bar halves from ﬂexural strength test were used for the compressive strength measurements.

2.2. Clay Bricks with Sand to OSA Replacement

2.2.1. Materials The clay (Cambrian blue clay) and clay/sand mixtures used in the green shaped bodies were obtained from Wienerberger Company—located in Aseri, Estonia. Similar physical and chemical characterization methods, as defined above for concrete application, have been used for the material characterization of the clay. The studied OSA sample for clay bricks is EPP-EPA obtained in August 2018 which was separated into different size fractions by size classification. The coarse fraction was used as sand or opening agent replacement in the formulation of clay bricks. Material characterization considered both the initial EPP-EPA and coarse fraction of EPP-EPA in terms of PSD, SSA, XRF and XRD analyses.

2.2.2. Sample Preparation In a first trial stage 3 brick formulations were tested. A reference brick was prepared with a mixture ratio of 80% clay/20% sand. A constant brick formulation of 80% clay/10% sand/10% EPP-EPA was used for both the initial and coarse ash. Bricks were prepared in triplicate by hand. The obtained fresh clay was first dried in a ventilated oven at 105 ◦C for 4 h. Dried agglomerates were comminuted using a ball mill for 20 min. Subsequently, the clay was ground to fine powder using a four-ball planetary mill (clockwise rotation for 10 min at 350 rpm, anti-clockwise rotation for 5 min at 350 rpm) and dried again at 105 ◦C for 4 h. The dried raw materials were mixed in the specified ratios and water was added to obtain similar plasticity (sight/feeling) of the mixtures. An infrared moisture analyzer was used to measure the water content of the mixtures. For EPP-EPA added bricks 20 wt. % water was used while 15 wt. % water was required for the reference bricks. The pastes were placed into 40 40 160 mm3 prisms and a hydraulic press applied a pressure of 75 kg/cm2 for 10 s. × × After demolding, the prepared green bodies were left to dry; first at room temperature (12 h) then in a drying chamber (with slow temperature ramp (~15 ◦C/h) up to 105 ◦C) until water evaporation related mass change stops before transferring them to the sintering stage. This process was carried Minerals 2020, 10, 765 4 of 19 out to avoid possible swelling or cracking of the samples in the furnace, caused by the expansion Minerals 2020, 10, x FOR PEER REVIEW 4 of 20 of entrapped water. Finally, the samples were sintered at 1020 ◦C which took approximately 48 h (includingevaporation slow heating related and mass cooling change cycle)stops before in an transf industrialerring them tunnel to furnacethe sintering under stage. oxidizing This process conditions. was carried out to avoid possible swelling or cracking of the samples in the furnace, caused by the 2.2.3. Testsexpansion and Measurements of entrapped water. Finally, the samples were sintered at 1020 °C which took Afterapproximately press molding, 48 h (including the samples slow wereheating weighed, and cooling and cycle) dimensions in an industrial were tunnel measured furnace to under calculate the oxidizing conditions. wet density. The same process was repeated after drying and sintering to obtain both dry density and end product2.2.3. Tests density and Measurements values. A water absorption test was carried out according to EN 771-1:2011 + A1:2015 [18].After The press flexural molding, and the compressive samples were strength weighed, testsand dimensions were performed were measured with Toni to calculate TechnikD-13355 the (Berlin,wet Brandenburg, density. The same Germany). process was The repeated thermal after conductivity drying and testsintering was to carried obtain outboth withdry density a HOT DISK TPS 2200and (Gothenburg, end product density Västra values. Götaland A water County, absorption Sweden) test was instrumentcarried out according which meetsto EN 771-1:2011 the ISO 22007-2 standard+ A1:2015 [19]. The[18]. intactThe flexural samples and compressive were used stre forngth this tests analysis. were performe The hotd with disk Toni Teflon TechnikD- sensor was placed on13355 opposite (Berlin, Brandenburg, intact sides Germany). of the bricks The andtherma thel conductivity thermal conductivity test was carried was out measured with a HOT at a rate DISK TPS 2200 (Gothenburg, Västra Götaland County, Sweden) instrument which meets the ISO of 10 K/s. The experiment is repeated on different sides of the same bricks and the mean value was 22007-2 standard [19]. The intact samples were used for this analysis. The hot disk Teflon sensor was recordedplaced for furtheron opposite calculation. intact sides of Porosity the bricks measurements and the thermal wereconductivity carried was out measured with a at POREMASTER a rate of PV00713010 K/s. (Graz, The Styria,experiment Austria is repeated) which on uses different three sides automatic of the same mercury bricks intrusionand the mean cycles value for was pore size analysis.recorded Measurements for further focusedcalculation. on Porosity intraparticle measurements porosity were which carried is theout porositywith a POREMASTER within individual grains orPV007130 particles. (Graz, Total Styria, porosity Austria) was which also uses measured three automatic to gain mercury insight intrusio in alln void cycles spaces for pore regardless size of interconnectivity.analysis. Measurements In order to focused investigate on intraparticle the microstructure porosity which of the is the sintered porosity bricks within SEM individual images were grains or particles. Total porosity was also measured to gain insight in all void spaces regardless of obtained from the polished samples using a ZEISS Evo MA 15 (Oberkochen, Baden-Württemberg, interconnectivity. In order to investigate the microstructure of the sintered bricks SEM images were Germany).obtained The from mineralogical the polished composition samples using was a ZEI determinedSS Evo MA using15 (Oberkochen, XRD analysis. Baden-Württemberg, Germany). The mineralogical composition was determined using XRD analysis. 2.3. Accelerated Carbonation of Compacted Samples 2.3. Accelerated Carbonation of Compacted Samples APP-EPA (October 2018) was chosen as starting material for the preliminary tests of compacted cylinders (FigureAPP-EPA1) as(October APP-EPA 2018) was is enriched chosen as starting in free material lime, therefore for the preliminary more suitable tests of compacted for carbonation cylinders (Figure 1) as APP-EPA is enriched in free lime, therefore more suitable for carbonation purposes. A representative sample was taken and divided into two size fractions (fine (0–100 µm) purposes. A representative sample was taken and divided into two size fractions (fine (0–100 µm) µ and coarseand coarse (100–300 (100–300m)) µm)) by sievingby sieving in in order order toto investigateinvestigate the the effect eff ectof particle of particle size on size compaction on compaction behaviorbehavior of ashes of ashes and extentand extent of carbonation.of carbonation.

FigureFigure 1. 1.100% 100% APP-EPA APP-EPA compacted compacted cylinders. cylinders.

2.3.1. Sample2.3.1. Sample Preparation Preparation The acceleratedThe accelerated carbonation carbonation process process consisted consisted of three of mainthree steps;main thesteps; ﬁrst the step first was step proportioning was proportioning of selected APP-EPA and prehydrating the sample to slake the free lime, the second of selected APP-EPA and prehydrating the sample to slake the free lime, the second step was the step was the compaction of the sample using a hydraulic press and the third step was the carbonation compactionof the product of the samplein an autoclave. using In a the hydraulic first step pressthe samples and were the thirdmixed stepwith water was theat a liquid carbonation to solid of the productmass in an ratio autoclave. of 0.25 in In a the semi-batch ﬁrst step Eirich the samplesEL1 (Hardheim, were mixed Baden-Württemberg, with water at aGermany) liquid to type solid mass ratio ofintensive 0.25 in amixer. semi-batch The samples Eirich were EL1 homogeneousl (Hardheim,y blended Baden-Württemberg, with water at a rotation Germany) speed type of 600 intensive mixer. The samples were homogeneously blended with water at a rotation speed of 600 rpm for

30 min. The samples were left to hydrate/cure in closed plastic bags overnight at room temperature and compacted on the following day using a hydraulic press into cylinders with diameter 20 mm and height 20 mm at a compaction pressure of 300 kg/cm2. Carbonation experiments were performed in automated Minerals 2020, 10, 765 5 of 19

Minerals 2020, 10, x FOR PEER REVIEW 5 of 20 carbonationrpm for units 30 min. consisting The samples of a temperaturewere left to hydrate/cure controlled in stainless-steel closed plastic bags autoclave. overnight The at autoclaveroom was operatedtemperature at two di andﬀerent compacted pressure on levelsthe following while temperatureday using a hydraulic stayed at press ambient into levels;cylinders the with compacted 2 specimensdiameter were 20 placedmm and in height the autoclave 20 mm at a andcompaction cured bypressure applying of 300 100% kg/cm CO. Carbonation2 gas pressure experiments of 5 and 10 bar for 4 h.were performed in automated carbonation units consisting of a temperature controlled stainless-steel autoclave. The autoclave was operated at two different pressure levels while temperature stayed at 2.3.2. Testsambient and levels; Measurements the compacted specimens were placed in the autoclave and cured by applying 100% CO2 gas pressure of 5 and 10 bar for 4 h. The obtained results from compressive strength measurements were used as main parameter 2.3.2. Tests and Measurements to evaluate the performance of the carbonated monoliths. The CO2 uptake during carbonation was determinedThe by obtained thermogravimetric results from compressive analysis strength (TGA). measurements Additionally, were XRD used measurements as main parameter were to made to followevaluate the formation the performance of new phasesof the carbonated during the monoliths. carbonation The process.CO2 uptake during carbonation was determined by thermogravimetric analysis (TGA). Additionally, XRD measurements were made to 3. Resultsfollow and the Discussionsformation of new phases during the carbonation process.

3.1. Oil3. ShaleResults Ash and Material Discussions Characterization The3.1. BETOil Shale specific Ash Material surface Characterization area (SSA) and the mean particle size of the OSAs considered in this paper are summarizedThe BET specific for surface APP (EPA,area (SSA) TA) and in Figure the mean2, andparticle for size EPP of (EPA, the OSAs TA) considered and EN280 in (TA,this CA) in Figurepaper3. The are variation summarized of the for physicalAPP (EPA, properties TA) in Figure is shown 2, and for EPP di ff (EPA,erent TA) sampling and EN280 dates. (TA, In CA) case of APP OSA, EPAin Figure shows 3. The consistently variation of finerthe physical particle properties sizes than is shown TA, even for different though sampling BET SSA dates. fall In within case ofthe same range.APP The OSA, EPP EPA EPA shows and consistently TA do not finer show particle this disizesfference than TA, in even particle though size BET and SSA are fall somewhat within the coarser same range. The EPP EPA and TA do not show this difference in particle size and are somewhat (AppendixA: Tables A1 and A2). The particle size of EN280-TA is coarser than EN280-CA and showed coarser (Appendix A: Tables A1 and A2). The particle size of EN280-TA is coarser than EN280-CA a correspondinglyand showed a correspondingly lower SSA. lower SSA.

Figure 2. Particle size and BET SSA for APP (EPA, TA). Minerals 2020, 10, x FOR PEERFigure REVIEW 2. Particle size and BET SSA for APP (EPA, TA). 6 of 20

Figure 3. Particle size and BET SSA for EPP (EPA, TA) and EN280 (TA, CA). Figure 3. Particle size and BET SSA for EPP (EPA, TA) and EN280 (TA, CA).

All OSAs studied mainly consist of SiO2, CaO and Al2O3, however in variable proportions (Figure 4). The main reason of such differences could be explained with several factors including characteristics of raw OS, OS processing conditions, thermochemical conversions, gaseous treatments for cleaning, boiler specifications, etc. In general, the high content of CaO can be explained by the decomposition of CaCO3 which originally exists in the mineral part of OS and partially decomposes during the thermal processes. The presence of SiO2 can be attributed to the inorganic part of the OS and mainly is representing finer particles as the content of silica is higher and lime content is lower in smallest size fractionated ashes. The chemical composition of OSAs differs and each type appears to show a distinct composition (Appendix B: Table A3). For instance, the high LOI of the EN280 residues is related to the lower temperature processing during retorting compared to combustion of OS causing delay in decomposition of carbonates and oxidation of unburnt carbon. Specifically, in case of EN280 TA, the high LOI of the residue correlates with a low SSA and coarse PSD which are all negative factors in terms of self-cementing behavior of these types of ashes especially when it is considered that fly ashes with high LOI can negatively affect the strength of the concrete (more water absorption, air- entraining, etc.) and increase setting times [20,21].

Figure 4. Ternary diagram of three major oxides in all OSAs.

Minerals 2020, 10, x FOR PEER REVIEW 6 of 20

Figure 3. Particle size and BET SSA for EPP (EPA, TA) and EN280 (TA, CA).

All OSAs studied mainly consist of SiO2, CaO and Al2O3, however in variable proportions (Figure 4). The main reason of such differences could be explained with several factors including characteristics of raw OS, OS processing conditions, thermochemical conversions, gaseous treatments for cleaning, boiler specifications, etc. In general, the high content of CaO can be explained by the decomposition of CaCO3 which originally exists in the mineral part of OS and partially decomposes during the thermal processes. The presence of SiO2 can be attributed to the inorganic part of the OS and mainly is representing finer particles as the content of silica is higher and lime content is lower in smallest size fractionated ashes. The chemical composition of OSAs differs and each type appears to show a distinct composition (Appendix B: Table A3). For instance, the high LOI of the EN280 residues is related to the lower Minerals 2020, 10, 765 6 of 19 temperature processing during retorting compared to combustion of OS causing delay in decomposition of carbonates and oxidation of unburnt carbon. Specifically, in case of EN280 TA, the high AllLOI OSAs of the studied residue mainlycorrelates consist with ofa low SiO 2SSA, CaO and and coarse Al2O PSD3, however which are in all variable negative proportions factors in (Figureterms of4 ).self-cementing The main reason behavior of such of these di ﬀ erencestypes of couldashes especially be explained when with it is several considered factors that including fly ashes characteristicswith high LOI of can raw negatively OS, OS processing affect the conditions, strength thermochemical of the concrete conversions, (more water gaseous absorption, treatments air- forentraining, cleaning, etc.) boiler and speciﬁcations, increase setting etc. times [20,21].

Figure 4. Ternary diagram of three majormajor oxidesoxides inin allall OSAs.OSAs.

In general, the high content of CaO can be explained by the decomposition of CaCO3 which originally exists in the mineral part of OS and partially decomposes during the thermal processes. The presence of SiO2 can be attributed to the inorganic part of the OS and mainly is representing finer particles as the content of silica is higher and lime content is lower in smallest size fractionated ashes. The chemical composition of OSAs differs and each type appears to show a distinct composition (AppendixB: Table A3). For instance, the high LOI of the EN280 residues is related to the lower temperature processing during retorting compared to combustion of OS causing delay in decomposition of carbonates and oxidation of unburnt carbon. Specifically, in case of EN280 TA, the high LOI of the residue correlates with a low SSA and coarse PSD which are all negative factors in terms of self-cementing behavior of these types of ashes especially when it is considered that fly ashes with high LOI can negatively affect the strength of the concrete (more water absorption, air-entraining, etc.) and increase setting times [20,21]. The variations in chemical composition are reflected in the phase composition as well as shown in Tables1–3. Some fluctuations in phase composition of the ashes were observed throughout the year in the EPA fractions. The amount of free CaO is higher than 10% for EPP and APP ashes (EPA, TA) and mineral CO2 content does not reach above 11%. Calcite and dolomite contents of EN280-CA and TAs are quite high, and the content of Ca/Mg silicates (C2S, akermanite, mervinite) and aluminates are quite low compared to all other studied samples due to the low temperature processing during retorting. Minerals 2020, 10, 765 7 of 19

Table 1. Phase composition of APP (EPA, TA).

APP EPA APP TA APP TA APP EPA APP EPA APP TA Phase 25.10.2018 25.10.2018 21.02.2018 15.02.2018 9.08.2018 10.07.2018 Quartz 12.6 9.9 12.2 15.5 13.1 12.2 K-feldspar 16.4 11.5 10.9 15.3 12.6 13.9 Mica 4.2 2.3 4.6 7.1 1.7 5.5 Calcite 13.5 20.7 23.1 17.0 12.7 15.6 Dolomite tr * 0.6 - - tr 1.7 Hematite 1.5 1.2 1.5 1.7 1.2 1.2 Lime 19.3 15.1 18.5 13.5 18.4 9.0 Portlandite 3.4 6.4 0.7 - 4.4 11.0 Periclase 3.0 4.3 2.7 2.8 2.8 2.8 Anhydrite 5.9 7.0 6.2 8.9 8.3 9.8 C2S 8 9 7.8 7.6 10.3 5.1 C4AF 3.1 3.6 3.3 3.4 4.5 1.9 Akermanite 1.9 1.8 3.5 2.6 2.3 2.3 Merwinite 4.1 4.1 4.0 3.2 2.6 5.4 Diopside 1.7 1.2 - - 2.8 - Sylvite 0.6 0.6 - 0.5 0.5 0.5 Wollastonite tr 0.7 - - 1.1 - * tr—Trace amount.

Table 2. Phase composition of EN280 (TA, CA).

EN280 CA EN280 TA EN280 CA EN280 TA EN280 CA EN280 TA EN280 TA Phase 30.10.2018 30.10.2018 19.03.2019 19.03.2019 12.07.2018 12.07.2018 09.02.2018 Quartz 14.8 3.0 15.5 3.3 16.8 4.3 6.8 K-feldspar 17.7 2.8 15.1 1.2 17.8 2.1 8.1 Mica 5.8 1.8 10.9 - 7.3 1.4 1.2 Calcite 28.6 60.0 30.5 73.8 31.4 69.2 63.6 Dolomite 2.4 16.6 3.6 5.1 3.2 7.0 3.0 Hematite 2.3 1.1 2.1 1.0 2.8 1.1 0.9 Lime 0.2 ------Portlandite tr tr - - - - - Periclase 2.8 2.8 2.8 2.8 2.5 2.2 1.5 Anhydrite 9.1 2.7 8.2 6.0 8.8 5.5 6.3 C2S 6.3 4.1 4.9 3.2 5.1 2.9 4.2 C4AF 3.5 1.9 2.3 2.3 2.1 1.0 2.1 Akermanite 1.4 1.4 0.8 1.1 1.0 1.8 0.8 Merwinite 1.4 1.0 1.0 tr 1.1 1.3 0.4 Diopside 2.5 0.7 1.6 - - - - Wollastonite tr 0.8 0.8 1.0 - - -

The samples that were used in last two sub-studies do not include ashes from oil shale retorting and these sub-studies only focus on the ashes produced from the combustion of oil shale. Both ashes used in the sub-studies are ﬂy ashes (70% of the total ash produced) collected from electrostatic precipitators where most of the ash is accumulated during the combustion of oil shale. By studying these ashes throughout the year, we noticed that the minerology, chemical and physical properties do not change drastically. Another important factor for selecting the ashes in the last two sub-studies, is the content of free CaO. Minerals 2020, 10, 765 8 of 19

Table 3. Phase composition of EPP (EPA, TA).

EPP EPA EPP EPA EPP TA Phase 15.5.2018 06.02.2018 06.02.2018 Quartz 18.1 19.9 17.0 K-feldspar 13.9 15.5 12.7 Mica 5.4 4.7 4.5 Calcite 15.1 8.1 11.1 Dolomite tr - - Hematite 2.1 2.4 2.0 Lime 7.0 12.7 14.0 Portlandite 2.2 - 1.5 Periclase 4.8 4.1 5.0 Anhydrite 9.7 11.4 11.0 C2S 9.2 4.9 5.6 C4AF 4.1 2.4 2.7 Akermanite 4.5 8.6 8.2 Merwinite 1.6 3.6 3.8 Sylvite 1.2 - - Wollastonite - 1.3 1.1

3.2. Self-Cementing Properties of Oil Shale Ash Self-cementing ability and setting times of the considered OSAs are shown in Figures5 and6. The self-cementing ability is aﬀected by the chemical composition and ﬁneness of ashes, in particular the content of free CaO, portlandite, Ca/Mg silicates and SO3 [22,23]. The self-cementing ability of ashes with coarse particle sizes and lowest temperature of treatment is generally lowest. This is demonstratedMinerals 2020, 10, x for FOR EN280-CA PEER REVIEW and TAs. 9 of 20

Figure 5. SSettingetting times and Self-cementitious ability for APP-EPA and APP-TA.

Figure 6. Setting times and self-cementitious ability for EPP (EPA, TA) and EN280 (TA, CA).

Because of this reason, less reactive ashes (EN280) would require post-treatments like higher temperature treatment, grinding, sieving etc. in order to increase their performance. Due to insufficient amounts of reactive phases present the EN280 ash specimens fell apart during the curing of samples as cohesion is lost after evaporation of water in the 60% RH environment (Figure 7).

Minerals 2020, 10, x FOR PEER REVIEW 9 of 20

Minerals 2020, 10, 765 9 of 19 Figure 5. Setting times and Self-cementitious ability for APP-EPA and APP-TA.

FigureFigure 6. 6. SettingSetting times times and and self-cementitious self-cementitious abilit abilityy for for EPP EPP (EPA, (EPA, TA) and EN280 (TA, CA).

BecauseComparing of this the self-cementingreason, less reactive properties ashes of (EN280) all studied would ashes, require the performance post-treatments of the like APP-EPA higher temperatureand APP-TA wastreatment, found togrinding, have better sieving properties. etc. in Between order to these increase two samples their performance. of ashes, APP-EPA Due to is insufficientshown to have amounts optimal of suitability. reactive phases This is present tentatively the EN280 explained ash byspecimens a higher BETfell apart SSA andduring a considerable the curing ofcontent samples of as lime cohesion and portlandite is lost after (~14–23%), evaporation Ca-Mg of water silicates in the 60% (C2S) RH and environment C4AF (~15–22%) (Figure that7). can participate in hydraulic or pozzolanic reactions. Therefore, APP-EPAs and EPP-EPAs have potential self-cementing properties. Setting times were tested according to EN 196-3:2016 in order to evaluate how long the samples remain in a plastic state that enable placing or casting the mixes. However, most of the studied mixes demonstrated very short setting times. This is tentatively related to the quick hydration of free CaO [24]. Because of this reason, less reactive ashes (EN280) would require post-treatments like higher temperature treatment, grinding, sieving etc. in order to increase their performance. Due to insuﬃcient amounts of reactive phases present the EN280 ash specimens fell apart during the curing of samples as cohesion is lost after evaporation of water in the 60% RH environment (Figure7). Minerals 2020, 10, x FOR PEER REVIEW 10 of 20

Figure 7. SpontaneousSpontaneous disintegration of EN280 specimens after 60% RH curing.

3.3. Clay Brick Production Using OSA It can be seen from Table 4 that EPP-EPA has more complex and different chemical composition than the clay which mostly consists of SiO2, Al2O3, Fe2O3 and K2O. The main oxides in EPP-EPA are SiO2—31.94%, CaO—35.25%, Al2O3, MgO and the content of SO3 is relatively high as well. The LOI is 4.8% due to the release of combined water, crystalline water, combustion of organic carbon and oxidation of sulfur.

Table 4. Chemical composition of EPP-EPA and clay.

Oxides EPP EPA (wt. %) Clay (wt. %) CaO total 35.25 0.4 MgO 5.85 2.3 SiO2 31.94 61.4 Al2O3 7.46 17.8 Fe2O3 - 5.9 SO3 7.12 1.6 K2O 3.32 6.1 Na2O 2.28 0.08 Mn2O3 0.06 - TiO2 0.38 - P2O5 0.14 - LOI * 4.82 4.8 TOC * 0.22 0.21 * LOI—Loss on ignition; TOC—Total organic carbon.

The mineralogy of the clay shows a heterogeneous mixture of minerals (Table 5) and can be subdivided into clay minerals (~60%) including kaolin, illite, illite-smectite, mica and chlorite, and non-clay minerals (~40%) including mainly quartz, orthoclase and minor constituents such as gypsum, pyrite and calcite. The clay used in this work is also characterized by a higher BET SSA— 30.86 m2/g and smaller mean particle size (15.1 µm).

Table 5. Phase composition of EPP-EPA and clay.

Phases EPP EPA (wt. %) Clay (wt. %) Quartz 16.1 27.2 Free Lime 12.6 - Anhydrite 9.1 - Calcite 8 0.3 Kaolin - 7.1 Periclase 3.8 - Hematite 2.9 - beta C2S 2 -

Minerals 2020, 10, 765 10 of 19

3.3. Clay Brick Production Using OSA It can be seen from Table4 that EPP-EPA has more complex and di ﬀerent chemical composition than the clay which mostly consists of SiO2, Al2O3, Fe2O3 and K2O. The main oxides in EPP-EPA are SiO2—31.94%, CaO—35.25%, Al2O3, MgO and the content of SO3 is relatively high as well. The LOI is 4.8% due to the release of combined water, crystalline water, combustion of organic carbon and oxidation of sulfur.

Table 4. Chemical composition of EPP-EPA and clay.

The mineralogy of the clay shows a heterogeneous mixture of minerals (Table5) and can be subdivided into clay minerals (~60%) including kaolin, illite, illite-smectite, mica and chlorite, and non-clay minerals (~40%) including mainly quartz, orthoclase and minor constituents such as gypsum, pyrite and calcite. The clay used in this work is also characterized by a higher BET SSA—30.86 m2/g and smaller mean particle size (15.1 µm).

Table 5. Phase composition of EPP-EPA and clay.

Phases EPP EPA (wt. %) Clay (wt. %) Quartz 16.1 27.2 Free Lime 12.6 - Anhydrite 9.1 - Calcite 8 0.3 Kaolin - 7.1 Periclase 3.8 - Hematite 2.9 - beta C2S 2 - gamma C2S 4.4 - C4AF 2.1 - Mullite 0 - Pyrite - 0.8 Ca-langbeinite 2.5 - Gypsum - 1.2 Chlorite - 5.1 Orthoclase 4.5 5.2 Illite, Illite-smectite, Mica 3.6 52.2 Amorphous 28.3 -

Based on the observations made on prepared samples, as a ﬁrst impression it can be mentioned that the color is one of the parameters to consider when producing bricks, as diﬀerent type of ashes Minerals 2020, 10, x FOR PEER REVIEW 11 of 20

gamma C2S 4.4 - C4AF 2.1 - Mullite 0 - Pyrite - 0.8 Ca-langbeinite 2.5 - Gypsum - 1.2 Chlorite - 5.1 Orthoclase 4.5 5.2 Illite, Illite-smectite, 3.6 52.2 Mica Amorphous 28.3 -

Minerals 2020, 10, 765 11 of 19 Based on the observations made on prepared samples, as a first impression it can be mentioned that the color is one of the parameters to consider when producing bricks, as different type of ashes wouldwould leadlead to to signiﬁcant significant changes changes in colorin color when when compared compared with thewith reference the reference bricks. Itbricks. can be It observed can be inobserved Figure8 in, EPP-EPA Figure 8, does EPP-EPA make does the color make of the bricks color lighter of bricks and lighter give a and new give yellowish a new color.yellowish color.

Figure 8.8. ((aa)) ReferenceReference 4:1 4:1 clay:sand, clay:sand, (b ()b 8:1:1) 8:1:1 clay:sand:EPP-EPA clay:sand:EPP-EPA coarse coarse and and (c) 8:1:1 (c) 8:1:1 clay:sand:EPP-EPA. clay:sand:EPP- EPA. The densities of bricks after molding, drying and sintering are presented in Table6. Dry densities are lower than wet densitiesTable and values6. Physical are properties proportional of sintered to the bricks. required initial water content for molding the bricks. The reaction between free CaO and water bonds water chemically which makes Physical Properties Reference EPP-EPA Initial EPP-EPA Coarse the dry density not exactly proportional to initial water added to the EPP-EPA added raw mixtures. Wet Density (kg/m3) 2138 1868 1818 It can also be seen that the values of reference bricks show a noteworthy increase in density after Dry Density (kg/m3) 1845 1779 1744 sintering while the density of EPP-EPA added bricks show only a slight increase. The shrinkage of Sintered density (kg/m3) 2185 1723 1766 bricks after sintering is given based on the volume changes and it can be noted that the reference Sintering shrinkage (%) 12 5 6 bricks have the highest shrinkage. With the addition of EPP-EPA, a notable reduction of shrinkage can be explained due to the different phase composition of EPP-EPA compared to sand which is mainly The densities of bricks after molding, drying and sintering are presented in Table 6. Dry densities inert and as a result accompanies different volume reduction. The presence of Ca(OH) , Ca and Mg are lower than wet densities and values are proportional to the required initial water2 content for carbonates, partially Ca- sulphate (the decomposition of sulphates can proceed well below 1000 C molding the bricks. The reaction between free CaO and water bonds water chemically which makes◦ in presence of CO [25]) in EPP-EPA strongly influences the brick microstructure by releasing H O, the dry density not exactly proportional to initial water added to the EPP-EPA added raw mixtures.2 CO and SO due to the new decomposition reactions attributed to EPP-EPA (Equations (1)–(4)) in It can2 also be2 seen that the values of reference bricks show a noteworthy increase in density after addition to clay mineral dehydroxylation, quartz inversion, crystallization and formation of vitreous sintering while the density of EPP-EPA added bricks show only a slight increase. The shrinkage of phase reactions [26]. bricks after sintering is given based on the volume changes and it can be noted that the reference Ca(OH) CaO + H O (1) bricks have the highest shrinkage. With the additi2 → on of EPP-EPA,2 a notable reduction of shrinkage can be explained due to the different CaCOphase compCaOosition+ CO of EPP-EPA compared to sand which(2) is 3 → 2 mainly inert and as a result accompanies different volume reduction. The presence of Ca(OH)2, Ca MgCO MgO+CO (3) and Mg carbonates, partially Ca- sulphate (the3 →decomposition2 of sulphates can proceed well below 1000 °C in presence of CO [25]) inCaSO EPP-EPA+ CO stronglyCaO influences+ SO + CO the brick microstructure by releasing(4) 4 → 2 2 H2O, CO2 and SO2 due to the new decomposition reactions attributed to EPP-EPA (Equations (1)–(4)) in addition to clay mineral Tabledehydroxylation, 6. Physical properties quartz ofinversion, sintered bricks. crystallization and formation of vitreous phase reactions [26]. Physical Properties Reference EPP-EPA Initial EPP-EPA Coarse Ca(OH)2 → CaO + H2O (1) Wet Density (kg/m3) 2138 1868 1818 3 Dry Density (kg/m ) 1845 1779 1744 Sintered density (kg/m3) 2185 1723 1766 Sintering shrinkage (%) 12 5 6

The physical and performance properties of the bricks are aﬀected by the increased porosity of the EPP-EPA bricks. The compressive and ﬂexural strength constitute the main parameters of brick performance. The obtained average value for the compressive strength EPP-EPA added bricks was ~20 MPa and for reference bricks ~30 MPa. These results correlate with the increased porosity of the EPP-EPA bricks as higher water absorption is usually associated with higher porosity as well. Water absorption values (Table7) showed lowest water absorption for the reference bricks and EPP-EPA addition led to an increase in water absorption value (2.5 times). Minerals 2020, 10, x FOR PEER REVIEW 12 of 20

CaCO3 → CaO + CO2 (2)

MgCO3 → MgO+CO2 (3)

CaSO4 + CO → CaO + SO2 + CO2 (4)

The physical and performance properties of the bricks are affected by the increased porosity of the EPP-EPA bricks. The compressive and flexural strength constitute the main parameters of brick performance. The obtained average value for the compressive strength EPP-EPA added bricks was ~20 MPa and for reference bricks ~30 MPa. These results correlate with the increased porosity of the EPP-EPA bricks as higher water absorption is usually associated with higher porosity as well. Water absorption values (Table 7) showed lowest water absorption for the reference bricks and EPP-EPA Mineralsaddition2020 led, 10 to, 765 an increase in water absorption value (2.5 times). 12 of 19

Table 7.7. Mechanical and thermal properties of bricks.

Compressive Flexural Water ThermalThermal Compressive Flexural Strength Water Absorption SampleSample Name Name Strength Strength Absorption ConductivityConductivity Strength (MPa) (MPa) (%) (MPa) (MPa) (%) (W/mK)(W/mK) ReferenceReference 30.630.6 8.98.9 5.25.2 1.22 EPP-EPAEPP-EPA initial initial 21.5 21.5 4.2 4.2 13.9 13.9 0.67 EPP-EPAEPP-EPA coarse coarse 19.3 19.3 4.4 4.4 13.3 13.3 0.71

Thermal propertiesproperties are are important important in termsin terms of heat of insulationheat insulation and energy and performanceenergy performance of buildings. of Thebuildings. thermal The conductivity thermal conductivity of a composite of a materialcomposite is determinedmaterial is determined by the properties by the of properties its constituents. of its Inconstituents. case of clay In bricks case theof thermalclay bricks conductivity the therma variesl conductivity depending varies on porosity depending and conductivity on porosity of and the solidconductivity constituents of the and solid exhibits constituents a decrease and exhibits in trend a with decrease bulk in density trend [with27]. bulk It can density be seen [27]. from It can the measuredbe seen from thermal the measured conductivity thermal values conductivity of the ﬁred valu bricks,es of giventhe fired in Tablebricks,7, thatgiven EPP-EPA in Table bricks7, that haveEPP- 50EPA % bricks better insulationhave 50 % propertiesbetter insulation than the properties reference than bricks. the reference bricks. An overall porosity reduction is expected when above 1000 ◦°CC the vitreous phase ﬁllsfills the pores and the ceramic body shrinks. From the porosity results shown in Figure9 9,, itit cancan bebe understoodunderstood thatthat the EPP-EPA samples samples show show a a higher higher volume volume of of pore pore features features in inthe the range range of 0–4 of 0–4 µmµ comparedm compared to the to thereference. reference.

Figure 9. Intraparticle porosity as a function of pore diameter size.

Additionally, the SEM SEM images images of of the the brick brick samples samples are are shown shown in Figure in Figure 10 and10 and it is itevident is evident that thata highera higher degreedegree of particle of particle interlocking, interlocking, and and mo morere homogeneous homogeneous texture texture can can be be observed observed for for the reference brickbrick microstructure.microstructure. The presence of carbonates strongly influences the brick microstructures and lowers the degree of shrinkage. This is partially due to the additional porosity generated during decomposition of the Ca carbonate into free lime and CO2 gas (Equation (2)). The CaO further recombines with the clay minerals during crystallization reactions and results in changes of the brick phase composition. Similar findings were also mentioned in other studies (Junge [28], Sütcü and Akkurt [29]) where other waste additives with high CaCO3 content (i.e., paper making sludge, limestone powder) were evaluated for brick production and these types of residues were identified as pore-forming additives. The phase composition of the EPP-EPA and the reference bricks is compared in Table8, the main difference is the presence of a significant fraction of plagioclase (Ca-feldspar) in the EPP-EPA bricks. In addition, small amounts of anhydrite and mullite were identified as well. Clearly CaO introduced by the EPP-EPA reacted with the aluminosilicates in the clay to form plagioclase feldspar. Anhydrite did not decompose fully during the sintering. Minerals 2020, 10, 765 13 of 19 Minerals 2020, 10, x FOR PEER REVIEW 13 of 20

FigureFigure 10. 10. TheThe SEM SEM images images of of th thee brick brick samples, samples, ( (aa)) Reference Reference sample, sample, ( (bb)) EPP-EPA EPP-EPA (coarse).

The presenceTable of carbonates 8. Phase composition strongly ofinfluences Reference the and brick EPP-EPA microstructures added bricks (coarse).and lowers the degree of shrinkage. This is partially due to the additional porosity generated during decomposition of the Ca carbonate into free lime Phaseand CO2 gas (Equation Reference (wt.(2)). %) The CaO EPP-EPA further (wt. recombines %) with the clay minerals during crystallizationQuartz reactions and results 58.0 in changes of 31.3 the brick phase composition. Similar findings were alsoK-feldspar mentioned in other studies 12.6 (Junge [28], Sütcü 20.8 and Akkurt [29]) where other Plagioclase - 17.7 waste additives with high CaCO3 content (i.e., paper making sludge, limestone powder) were Mica 3.6 6.03 evaluated for brick productionHematite and these types of 3.8residues were identified 4.5 as pore-forming additives. The phase compositionAnhydrite of the EPP-EPA and the - reference bricks is 2.1 compared in Table 8, the main difference is the presence of aC significant2S fraction of - plagioclase (Ca-feldspar) 1.3 in the EPP-EPA bricks. In addition, small amounts ofC4 anhydriteAF and mullite were identified as 0.5 well. Clearly CaO introduced by the EPP-EPA reacted withSanidine the aluminosilicates 9.7 in the clay to form plagioclase - feldspar. Anhydrite Spinel 11.8 10.2 did not decompose fully duringMullite the sintering. - 3.7

Table 8. Phase composition of Reference and EPP-EPA added bricks (coarse). 3.4. Carbonate Bound Monolith Production from Oil Shale Ash Phase Reference (wt. %) EPP-EPA (wt. %) CaO/MgO and Ca/Mg-silicates in OSA are potential phases for CO sequestration via carbonation. Quartz 58.0 31.3 2 By carbonation of compacted samples, CO can be permanently stored as Ca or Mg carbonates. K-feldspar 12.62 20.8 The formation of carbonates is associated with an increase in solid volume and a decrease in porosity Plagioclase - 17.7 of the compacts. The carbonates act as cement by forming solid bridges between reacting particles and Mica 3.6 6.03 inﬁlling of porosity. Hematite 3.8 4.5 The compressive strength test results of the compacted APP-EPAs are quite promising and Anhydrite - 2.1 compacts that are made of coarser fraction have greater compressive strength (up to 41.4 MPa) for C2S - 1.3 both tested pressure levels (Table9). Interestingly, the compacts carbonated at 5 bar show higher C4AF 0.5 strength values compared to 10 bar. This may suggest that high pressures lead to fast reactions causing Sanidine 9.7 - pore clogging near the surface of the compacts and zonation instead of homogeneous carbonation. Spinel 11.8 10.2 In fact, previous studies have also shown that excess CO pressure does not necessarily lead to a higher Mullite - 2 3.7 compressive strength as slower reaction would allow for dissipation of heat and reduce stresses on the product [30]. Further research into the microstructure of the carbonated compacts is needed to verify the mechanism controlling the carbonation reaction and to further optimize the performance of the carbonated products.

Minerals 2020, 10, 765 14 of 19

Table 9. Compressive strength, water absorption and density results for carbonated APP EPA compacts.

Particle Size (µm) and Compressive Strength Density Water Pressure (bar) (MPa) (kg/m3) Absorption (%) 0–100 µm 24.9 4.5 1992 11.2 5 bar ± 0–100 µm 8.6 1.3 1807 12.3 10 bar ± 100–300 µm 38.1 1.9 1882 9.8 5 bar ± 100–300 µm 20.1 1.6 1675 10.4 10 bar ±

Thermogravimetric analysis measurements of the compacts made from coarse fraction of APP EPA are presented in Figure 11. The mass loss due to the release of CO2 that is related to decomposition of carbonates is indicated for the uncarbonated sample as well as it already includes CaCO3 in the raw untreated ash which is measured as 6%. The samples cured at 5 and 10 bar show higher mass loss which is related to the mineral CO2 bounded during carbonation, indicating that the CO2 uptake during the carbonation treatment can be up to ~9% of the total mass of the sample. Minerals 2020, 10, x FOR PEER REVIEW 15 of 20

Figure 11. TGA curves for monoliths made from coarse fraction of APP EPA.

The phase composition of the uncarbonated and and carbonated carbonated compacts compacts are are given given in in Table Table 10.10. Portlandite Ca(OH)2 and ettringite are major phases presentpresent inin thethe uncarbonateduncarbonated sample.sample. In the carbonated samplessamples a a strong strong increase increase in in the the calcite calcite (CaCO (CaCO3) fraction3) fraction is notable. is notable. Calcite Calcite appears appears to form to mainlyform mainly at the expenseat the expense of portlandite of portlandite and ettringite. and ettr Additionalingite. Additional to calcite, to some calcite, gypsum some has gypsum formed has by carbonationformed by carbonation of ettringite. of Also,ettringite. C2S and Also, merwinite C2S and merwinite appear to haveappear partially to have carbonated. partially carbonated. As noticed As as wellnoticed in the as well TGA, in portlandite the TGA, portlandite is not fully is consumed not fully indicatingconsumed thatindicating further that improvement further improvement of the process of isthe possible. process Theis possible. fine ash fractionThe fine of ash 0–100 fractionµm shows of 0–100 differences µm shows in its initialdifferences phase in composition its initial phase being highercomposition in quartz, being K-feldspar higher in and quartz, mica K-feldspar and lower an ind anhydrite mica and phases. lower Thisin anhydrite is reflected phases. in the This phase is compositionreflected in the of thephase carbonated compositio material.n of the carbonated material.

Table 10. Phase composition of monoliths.

100–300 μm 0–100 μm 100–300 μm 100–300 μm Phase Uncarbonated 5 bar 5 bar 10 bar Quartz 5.4 12.7 4.6 5 K-feldspar 1.5 14.4 1.7 1.4 Mica - 3.1 - - Calcite 10.2 43.5 35.1 38.4 Dolomite 1.9 3.1 2.7 3.6 Anhydrite 16.1 5.9 16.9 16.2 Periclase 2.4 2.6 3.6 3.9 Portlandite 36.1 trace 17.2 15.4 Vaterite - 0.5 - - Akermanite 1.8 4 2.2 1.6 Merwinite 3 0.5 1.5 1.5 C2S 4.7 0.8 1.6 1.3 Hematite 1.1 2.2 1 0.8 Ettringite 10.1 1.9 5.7 4.5 Gypsum 0.7 4.1 2.9 3.9 Brucite 4.7 trace 3 2.5

Minerals 2020, 10, 765 15 of 19

Table 10. Phase composition of monoliths.

100–300 µm 0–100 µm 100–300 µm 100–300 µm Phase Uncarbonated 5 bar 5 bar 10 bar Quartz 5.4 12.7 4.6 5 K-feldspar 1.5 14.4 1.7 1.4 Mica - 3.1 - - Calcite 10.2 43.5 35.1 38.4 Dolomite 1.9 3.1 2.7 3.6 Anhydrite 16.1 5.9 16.9 16.2 Periclase 2.4 2.6 3.6 3.9 Portlandite 36.1 trace 17.2 15.4 Vaterite - 0.5 - - Akermanite 1.8 4 2.2 1.6 Merwinite 3 0.5 1.5 1.5 C2S 4.7 0.8 1.6 1.3 Hematite 1.1 2.2 1 0.8 Ettringite 10.1 1.9 5.7 4.5 Gypsum 0.7 4.1 2.9 3.9 Brucite 4.7 trace 3 2.5

4. Conclusions and Perspectives Estonian oil shale ash (OSA) characterization results showed that the new types of OSA from Enefit280 units contain significantly less free lime and more undecomposed carbonates compared to other combustion technologies. Additionally, chemical and physical composition of this type of ash is quite variable throughout the testing period of this study. In contrast, both the chemical and physical composition of APP and EPP ashes stay practically unchanged providing easier implementation of storage or recycling treatments. APP-EPA is the only type of ash which can be considered as an independent potential binder. The rest of the tested ashes would require additional physical or thermal pre-treatments to make them fit for use in cement and concrete applications. These types of wastes for instance could be re-evaluated as a composite raw material for mixed binders including other constituents. The effect of EPP-EPA addition was investigated in terms of physical, mechanical and thermal properties of clay bricks. From a broad perspective it can be concluded that the physical, mechanical and thermal properties (i.e., workability of raw mixtures, water absorption, color, texture, density, porosity, strength, and thermal conductivity) of the produced bricks are affected by the composition of the OSA. There is a continuous effect of the exothermic CaO–H2O reaction (forming Ca(OH)2) being active as soon as raw materials are in contact with water and prolonged even after press molding which may cause slight expansion and higher porosity already before the firing stage. Further, release of bound water due to the decomposition of hydrates and decomposition of Ca(OH)2, CaCO3 and partially CaSO4 lead to the creation of additional porosity during the firing process. In this respect, EPP-EPA and similar OSAs or carbonate rich wastes with high reactive free lime content can be considered as pore forming agent for clay bricks and applications require specific care to control the product performances. From a technical point of view, the results obtained from accelerated carbonation of compacted APP-EPAs demonstrate that this type of compaction and carbonation process can be a promising way of waste utilization as it provides a way to produce valuable construction materials with high material strength while also binding CO2. In this context, efforts and studies should proceed to further develop and optimize this process for OSA in order to improve the extent of the carbonation and produce specific building materials, for instance with good thermal and sound insulation characteristics. To conclude, the results presented in this study indicate potentially interesting pathways for OSA utilization and all observations provide a basis of the future optimization of OS combustion and treatment processes in view of utilization of the generated ashes. Minerals 2020, 10, 765 16 of 19

Author Contributions: Conceptualization, M.C.U., C.R.Y. and R.S.; data curation, P.P.; formal analysis, M.U.; investigation, T.H.; methodology, A.G.; resources, E.R.; validation, R.K. and A.T. All authors have read and agreed to the published version of the manuscript. Funding: This work is partly supported by the CLEANKER project which has received funding from the European Union’s Horizon 2020 Framework Program under Grant Agreement No. 764816 and from the China Government (National Natural Science Foundation of China) under contract No. 91434124 and No. 51376105. This study is also supported by the Estonian Ministry of Education and Research (IUT33-19), the European Regional Development Fund (RITA1/01-01-07), and Eesti Energia AS. Additionally, the activities related to clay bricks have received funding from EIT Raw Materials a body of the EU under Horizon 2020, under the grant name Flame: Fly ash to valuable minerals. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations

OSA Oil Shale Ash CFB Circulating fluidized bed PF Pulverized firing EPA Electrostatic precipitator ash CA Cyclone ash TA Total ash FA Fly ash EPP Estonian power plant APP Auvere power plant EN280 Enefit 280 shale oil plant PSD Particle size distribution SSA Specific surface area XRF X-ray fluorescence XRD X-ray diffraction TGA Thermogravimetric analysis

Appendix A

Table A1. Particle size distribution of EPP and EN280 ashes (d50, d90, d10).

Name Type Date d50 d90 d10 EPP EPA 02.06.2018 37.3 74.2 10.1 EPP EPA 05.15.2018 42.8 74.2 10.1 EPP TA 02.06.2018 44.0 89.5 10.6 EN280 TA 02.09.2018 280.8 934.5 8.1 EN280 TA 07.12.2018 36.9 279.4 8.1 EN280 TA 10.30.2018 28.6 279.4 8.1 EN280 TA 03.19.2019 31.2 279.4 8.1 EN280 CA 07.12.2018 22.5 74.2 10.1 EN280 CA 10.30.2018 21.3 74.2 10.1 EN280 CA 03.19.2019 34.7 74.2 10.1

Table A2. Particle size distribution of APP ashes (d50, d90, d10).

Name Type Date d50 d90 d10 APP EPA 02.15.2018 21.9 57.1 7.7 APP EPA 05.12.2018 23.1 42.3 7.1 APP EPA 08.09.2018 23.0 44.2 8.9 APP EPA 10.25.2018 22.0 45.3 8.4 APP EPA 03.13.2019 23.2 75.3 7.7 APP TA 02.21.2018 31.4 73.1 7.7 APP TA 05.12.2018 32.9 51.2 7.1 APP TA 07.10.2018 28.7 64.7 8.9 APP TA 10.25.2018 34.2 65.2 7.7 APP TA 03.13.2019 37.6 75.7 8.4 Minerals 2020, 10, 765 17 of 19

Appendix B

Table A3. Chemical composition of all oil shale ashes.

Component SiO2 Al2O3 TiO2 Fe2O3 MnO CaO MgO Na2OK2OP2O5 SO3 LOI Unit mass% mass% mass% mass% mass% mass% mass% mass% mass% mass% mass% mass% APP TA 25.10.2018 20.5 4.8 0.2 2.9 0.1 42.4 3.8 0.1 2.5 0.2 5.2 15.0 APP EPA 25.10.2018 27.1 6.3 0.3 3.1 0.0 39.3 2.9 0.4 3.1 0.1 3.8 12.5 APP EPA 13.03.2018 22.0 5.2 0.2 3.6 0.1 39.1 7.2 0.3 2.2 0.1 6.7 11.2 APP TA 13.03.2018 22.3 5.2 0.2 3.2 0.1 40.3 5.5 0.1 2.5 0.1 6.5 12.2 APP EPA 09.08.2018 26.6 6.3 0.3 3.2 0.0 38.9 2.8 0.2 3.0 0.1 4.3 12.6 APP TA 12.05.2018 17.3 4.0 0.2 2.7 0.1 43.1 5.1 0.1 2.1 0.2 5.0 17.8 EPP EPA 06.02.2018 35.8 11.3 0.7 5.5 0.1 27.3 4.4 0.1 4.5 0.1 5.6 4.0 EPP TA 06.02.2018 30.8 9.6 0.6 5.2 0.1 32.2 4.9 0.1 3.7 0.1 5.6 6.5 EPP EPA 15.05.2018 33.2 7.8 0.4 4.0 0.1 30.0 4.9 0.2 3.5 0.1 4.8 8.3 EN280 TA 19.03.2019 6.7 1.4 0.1 1.8 0.0 48.1 3.2 0.1 0.6 0.1 4.1 34.5 EN280 CA 19.03.2019 29.9 7.0 0.3 3.7 0.0 29.0 3.4 0.1 3.5 0.1 4.5 17.3 EN280 TA 30.10.2018 5.2 1.2 0.1 1.4 0.1 49.0 3.3 0.1 0.4 0.1 2.3 37.7 EN280 CA 30.10.2018 28.3 6.7 0.3 3.6 0.1 29.2 3.6 0.2 3.3 0.1 4.7 18.0 Minerals 2020, 10, 765 18 of 19

References

1. Publications Oﬃce of the European Union. Best Available Techniques (Bat) Reference Document for Waste Treatment Industrial Emissions Directive 2010/75/Eu (Integrated Pollution Prevention and Control). Available online: https://op.europa.eu/en/publication-detail/-/publication/782f0042-d66f-11e8- 9424-01aa75ed71a1/language-en (accessed on 10 May 2020). 2. National Development Plan of Estonia 2016–2030. Available online: https://www.envir.ee/sites/default/ﬁles/ 2016_2030ak_ingl.pdf (accessed on 10 May 2020). 3. Estonian Oil Shale Industry Yearbook 2018. Available online: https://www.ttu.ee/public/p/polevkivi- kompetentsikeskus/aastaraamat/Polevkivi_aastaraamat_2019_ENG_veeb.pdf (accessed on 11 May 2020). 4. Kuusik, R.; Uibu, M.; Kirsimäe, K. Characterization of oil shale ashes formed at industrial-scale boilers. Oil Shale 2005, 22, 407–420. 5. Blinova, I.; Bityukova, L.; Kasemets, K.; Ivask, A.; Kakinen, A.; Kurvet, I.; Bondarenko, O.; Kanarbik, L.; Sihtmäe, M.; Aruoja, V.; et al. Environmental Hazard of Oil Shale Combustion Fly Ash. J. Hazard. Mat. 2012, 229–230, 192–200. [CrossRef][PubMed] 6. Kahru, A.; Polluma, L. Environmental Hazard of the Waste Streams of Estonian Oil Shale Industry: An Ecotoxicological Review. Oil Shale 2006, 23, 53–93. 7. Kuusik, R.; Uibu, M.; Kirsimäe, K.; Mõtlep, R.; Meriste, T. Open-air Deposition of Estonian Oil Shale Ash: Formation, State of Art, Problems and Prospects for the Abatement of Environmental Impact. Oil Shale 2012, 29, 376–403. [CrossRef] 8. Mohammad, M.S.; Rami, H.H. The Use of Oil Shale Ash in Portland Cement Concrete. Cem. Concr. Compos. 2003, 25, 43–50. 9. Gomes, H.I.; Mayes, W.M.; Rogerson, M.; Stewart, D.I.; Burke, I.T. Alkaline Residues and the Environment: A Review of Impacts, Management Practices and Opportunities. J. Clean. Prod. 2016, 112, 3571–3582. [CrossRef] 10. Hadi, N.A.R.A.; Abdelhadi, M. Characterization and Utilization of Oil Shale Ash Mixed with Granitic and Marble Wastes to Produce Lightweight Bricks. Oil Shale 2018, 35, 56–69. [CrossRef] 11. Gunning, P.J.; Hills, C.D.; Carey, P.J. Production of Lightweight Aggregate from Industrial Waste and Carbon Dioxide. Waste Manag. 2009, 29, 2722–2728. [CrossRef] 12. Quaghebeur, M.; Nielsen, P.; Horckmans, L.; Van Mechelen, D. Accelerated Carbonation of Steel Slag Compacts: Development of High-Strength Construction Materials. Front. Energy Res. 2015, 3, 52. [CrossRef] 13. Nielsen, P.; Boone, M.A.; Horckmans, L.; Snellings, R.; Quaghebeur, M. Accelerated Carbonation of Steel

Slag Monoliths at Low CO2 Pressure–Microstructure and Strength Development. J. CO2 Utiliz. 2020, 36, 124–134. [CrossRef] 14. Raado, L.; Kuusik, R.; Hain, T.; Uibu, M.; Somelar, P. Oil Shale Ash Based Stone Formation—Hydration, Hardening Dynamics And Phase Transformations. Oil Shale 2014, 31, 91. [CrossRef] 15. EESTI Standard. Methods of Testing Cement—Part 1: Determination of Strength. Available online: https://www.evs.ee/preview/evs-en-196-1-2016-en.pdf (accessed on 12 May 2020). 16. EESTI Standard. Metallic Materials—Calibration and Verification of Static Uniaxial Testing Machines—Part 1:Tension/Compression Testing Machines—Calibration and Verification of the Force-Measuring System. Available online: https://www.evs.ee/tooted/evs-en-iso-7500-1-2018 (accessed on 12 May 2020). 17. ISO 679:2009. Cement-Test Methods-Determination of Strength. Available online: https://www.iso.org/ standard/45568.html (accessed on 12 May 2020). 18. EESTI Standard. Specification for Masonry Units—Part 1: Clay Masonry Units. Available online: https: //www.evs.ee/en/evs-en-771-1-2011%2Ba1-2015 (accessed on 12 May 2020). 19. International Standard. Transient Plane Heat Source (Hot Disc) Method. Available online: https://www.evs. ee/products/iso-22007-2-2015 (accessed on 12 May 2020). 20. Dhir, R.K.; Munday, J.G.L.; Ong, L.T. Strength variability of OPC/PFA concrete. Concrete 1981, 15, 33–37. 21. Das, S.K.; Yudhbir. A Simplified Model for Prediction of Pozzolanic Characteristics of Fly Ash, Based on Chemical Composition. Cement Concr. Res. 2006, 36, 1827–1832. [CrossRef] 22. Pihu, T.; Konist, A.; Puura, E.; Liira, M.; Kirsimäe, K. Properties and Environmental Impact of Oil Shale Ash Landfills. Oil Shale 2019, 36, 257. [CrossRef] Minerals 2020, 10, 765 19 of 19

23. Pihu, T.; Arro, H.; Prikk, A.; Rootamm, R.; Konist, A.; Kirsimäe, K.; Mõtlep, R. Oil shale CFBC ash cementation properties in ash fields. Fuel 2012, 93, 172–180. [CrossRef] 24. Raado, L.; Hain, T.; Liisma, E.; Kuusik, R. Composition and Properties of Oil Shale Ash Concrete. Oil Shale 2014, 31, 147. [CrossRef] 25. Yörük, C.R.; Meriste, T.; Trikkel, A.; Kuusik, R. Thermo-Oxidation Characteristics of Oil Shale and Oil Shale Char Under Oxy-Fuel Combustion Conditions. J. Therm. Anal. Calorim. 2015, 121, 509–516. [CrossRef] 26. Ferrer, S.; Mezquita, A.; Gomez-Tena, M.P.; Machi, C.; Monfort, E. Estimation of The Heat of Reaction In Traditional Ceramic Compositions. Appl. Clay Sci. 2015, 108, 28–39. [CrossRef] 27. Kubi´s,M.; Pietrak, K.; Cie´slikiewicz,L.; Furmański,P.; Wasik, M.; Seredyński,M.; Wi´sniewski,T.S.; Łapka, P. On the Anisotropy of Thermal Conductivity in Ceramic Bricks. J. Build. Eng. 2020, 31, 101418. [CrossRef] 28. Junge, K. Additives in the Brick and Tile Industry. Ziegelind. Int. 2000, 53, 25–39. 29. Sütcü, M.; Akkurt, S. The Use of Recycled Paper Processing Residues in Making Porous Brick with Reduced Thermal Conductivity. Ceram. Int. 2009, 35, 2625–2631. [CrossRef] 30. Fernandezbertos, M.; Simons, S.; Hills, C.; Carey, P. A Review of Accelerated Carbonation Technology in

the Treatment of Cement-Based Materials and Sequestration of CO2. J. Hazard. Mat. 2004, 112, 193–205. [CrossRef][PubMed]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).